When he is eating on the breast for those 5 minutes, is he drinking and swallowing the whole time? If he is, then no worries, if you have a good milk supply, then that's probably enough for him. He's obviously getting enough food, my 1 year old is only a little over 19 pounds, so your baby definitely has been eating :) Have you actually been giving formula after every breastfeeding, so every 1 or 2 hours? I bet he doesn't need that. At that age, I could imagine him eating every 1-2 hours (some babies do go longer, but all are different), and if you are feeding him formula because you think you have a supply issue, you really don't need to. If you just feed him from the breast on demand, if you aren't making enough milk (although I really think you have no problem with that), you will start to produce more because he will start feeding more (maybe even longer). Maybe he is only on for 10 minutes a feeding because he's just snacking. If you take a couple of days, breastfeed him only, and on demand, you will probably find that he doesn't really need that formula, and may even start to eat longer. One more thing, when you feed him every 1 to 2 hours, is he really hungry, or fussy about something else? Try diaper, nap, etc. before you feed him, since he might not actually be hungry every time. This may help him snack less, if that is what he is doing.